How does the Herpes Simplex virus develop when neither partner has the virus?

It doesn't just "develop". One of you have it, you probably just don't know it.

Also, what kind of herpes are you talking about? There are two herpes simplex viruses. Herpes simplex virus 1 are cold sores and herpes simplex virus 2 are genital herpes.

Um, the problem is that it doesn't, a virus needs a host to replicate itslef...heres a link about viruses in general

Genital herpes is an infection of the genitals, buttocks, or anal area caused by herpes simplex virus (HSV). There are two types of HSV.
HSV type 1 most commonly infects the mouth and lips, causing sores known as fever blisters or cold sores. It is also an important cause of sores to the genitals.
HSV type 2 is the usual cause of genital herpes, but it also can infect the mouth.
If you have genital herpes infection, you can easily pass or transmit the virus to an uninfected partner during sex.

Most people get genital herpes by having sex with someone who is shedding the herpes virus either during an outbreak or during a period with no symptoms. People who do not know they have herpes play an important role in transmission.

You can transmit herpes through close contact other than sexual intercourse, through oral sex or close skin-to-skin contact, for example.

The virus is spread rarely, if at all, by objects such as a toilet seat or hot tub.

Reduce your risk of spreading herpes

People with herpes should follow a few simple steps to avoid spreading the infection to other places on their body or other people.

Avoid touching the infected area during an outbreak, and wash your hands after contact with the area.
Do not have sexual contact (vaginal, oral, or anal) from the time of first genital symptoms until symptoms are completely gone.
Symptoms of herpes are called outbreaks. The first outbreak appears within 2 weeks after you become infected and can last for several weeks. These symptoms might include tingling or sores near the area where the virus has entered the body, such as on the genital or rectal area, on buttocks or thighs, or occasionally on other parts of the body where the virus has entered through broken skin. They also can occur inside the vagina and on the cervix in women, or in the urinary passage of women and men. Small red bumps appear first, develop into small blisters, and then become itchy, painful sores that might develop a crust and will heal without leaving a scar.

Sometimes, there is a crack or raw area or some redness without pain, itching, or tingling.

Other symptoms that may accompany the first (and less often future) outbreak of genital herpes are fever, headache, muscle aches, painful or difficult urination, vaginal discharge, and swollen glands in the groin area.

Often, though, people don鈥檛 recognize their first or subsequent outbreaks. People who have mild or no symptoms at all may not think they are infected with herpes. They can still transmit the virus to others, however.

Recurrence of herpes outbreaks

In most people, the virus can become active and cause outbreaks several times a year. This is called a recurrence, and infected people can have symptoms. HSV remains in certain nerve cells of your body for life. When the virus is triggered to be active, it travels along the nerves to your skin. There, it makes more virus and sometimes new sores near the site of the first outbreak.

Recurrences are generally much milder than the first outbreak of genital herpes. HSV-2 genital infection is more likely to result in recurrences than HSV-1 genital infection. Recurrences become less common over time.

Symptoms from recurrences might include itching, tingling, vaginal discharge, and a burning feeling or pain in the genital or anal area. Sores may be present during a recurrence, but sometimes they are small and easily overlooked.

Sometimes, the virus can become active but not cause any visible sores or any symptoms. During these times, small amounts of the virus may be shed at or near places of the first infection, in fluids from the mouth, penis, or vagina, or from barely noticeable sores. This is called asymptomatic (without symptoms) shedding. Even though you are not aware of the shedding, you can infect a sexual partner during this time. Asymptomatic shedding is an important factor in the spread of herpes